Add expression language reference (#331)

* Add expression language doc

* update mixer config with el doc ref

* minor fix

* remove include home

* review comments

* relocate expression doc

* removed details

_docs/concepts/policy-and-control/mixer-config.md

@@ -251,15 +251,7 @@ a particular label is of type INT64, then only attribute expressions that produc
 64-bit integer can be used to fill-in that label. This is the case for the `responseCode`
 label above.
 
-Attribute expressions include the following features:
-
-1. Check attributes for equality against constants
-2. Check string attributes for wildcard matches
-3. Logical AND/OR/NOT operators
-4. Grouping semantics
-5. String Concatenation
-6. Substring Production
-7. Comparison (<, <=, ==, >=, >)
+Refer to the [attribute expression reference]({{home}}/docs/reference/config/mixer/expression-language.html) for details.
 
 #### Selectors
 

_docs/reference/config/mixer/expression-language.md

@@ -0,0 +1,61 @@
+---
+title: Expression Language
+overview: Mixer config expression language reference.
+
+order: 43
+
+layout: docs
+type: markdown
+---
+
+{% capture mixerConfig %}{{home}}/docs/concepts/policy-and-control/mixer-config.html{% endcapture %}
+
+This page describes how to use the Mixer config expression language (CEXL).
+
+## Background
+
+Mixer configuration uses an expression language (CEXL) to specify [selectors]({{mixerConfig}}#selectors) and [mapping expressions]({{mixerConfig}}#attribute-expressions). CEXL expressions map a set of typed [attributes]({{home}}/docs/concepts/policy-and-control/attributes.html) and constants to a typed [value](https://github.com/istio/api/blob/master/mixer/v1/config/descriptor/value_type.proto#L23).
+  
+## Syntax
+
+CEXL accepts a subset of **[Go expressions](https://golang.org/ref/spec#Expressions)**, which defines the syntax. CEXL implements a subset of the Go operators that constrains the set of accepted Go expressions. CEXL also supports arbitrary parenthesization.
+
+## Functions
+
+CEXL supports the following functions.
+
+|Operator |Description |Example 
+|------------------------------------
+|`==` |Equals |`request.size == 200` 
+|`==` |Equals Prefix|`service.name == "svc1.*"` 
+|`==` |Equals Suffix|`service.name == "*.ns1.svc.cluster.local"` 
+|`!=` |Not Equals |`request.user != "admin"`
+|`||` |Logical OR | `(request.size == 200) || (request.user == "admin")` 
+|`&&` |Logical AND | `(request.size == 200) && (request.user == "admin")` 
+|`[ ]` |Map Access | `request.headers["x-id"]`
+|`|` |First non empty | `source.labels["app"] | source.labels["svc"] | "unknown"`
+
+## Type checking
+
+CEXL variables are attributes from the typed [attribute vocabulary]({{home}}/docs/reference/config/mixer/attribute-vocabulary.html), constants are implicitly typed and, functions are explicitly typed.
+
+Mixer validates a CEXL expression and resolves it to a type during config validation.
+Selectors must resolve to a boolean value and mapping expressions must resolve to the type they are mapping into. Config validation fails if a selector fails to resolve to a boolean or if a mapping expression resolves to an incorrect type. 
+
+For example, if an operator specifies a *string* label as `request.size | 200`, validation fails because the expression resolves to an integer.
+
+## Missing attributes
+
+If an expression uses an attribute that is not available during request processing, the expression evaluation fails. Use the `|` operator to provide a default value if an attribute may be missing. 
+
+For example, the expression `request.user == "user1"` fails evaluation if the request.user attribute is missing. The `|` (OR) operator addresses the problem: `(request.user | "nobody" ) == "user1"`.
+
+## Examples
+
+|Expression |Return Type |Description
+|------------------------------------
+|`request.size| 200` |  **int** | request.size if available, otherwise 200.
+|`request.header["X-FORWARDED-HOST"] == "myhost"`| **boolean** 
+|`(request.header["x-user-group"] == "admin") || (request.user == "admin")`| **boolean**| True if the user is admin or in the admin group.
+|`(request.user | "nobody" ) == "user1"` | **boolean** | True if request.user is "user1", The expression will not error out if request.user is missing.
+|`source.labels["app"]=="reviews" && source.labels["version"]=="v3"`| **boolean** | True if app label is reviews and version label is v3, false otherwise.